Disk drives are widely used not only in computer apparatus but also in home electrical appliances such as hard disk recorders. A disk drive has a recording medium and a head. The head flies above the recording medium to record or reproduce data in or from the same data by magnetizing the recording medium or reading the state of magnetization of the recording medium.
Recently, there is increasing demand for variable-speed disk drives capable of switching disk operating conditions between a plurality of rotating speeds (for example, see JP-A-2003-006990 (Patent Document 1). For example, in the field of disk drives used for recording and reproducing movies or images associated with news, there is a need for rotating a recording medium at a high speed to complete transfer in a short time when data are transferred from a server at a shop or home to a portable reproduction apparatus. There is also a need for rotating a recording medium at a low speed to suppress power consumption and noises when images are actually reproduced and watched in a mobile environment in which not so high data processing speed is required. In the case of disk drives for portable video cameras, for example, there is a need for rotating a recording medium at a low speed to suppress power consumption and noises during recording in a mobile environment and for rotating a recording medium at a high speed to complete transfer in a short time in transferring data to a stationary recorder or computer.
A magnetic disk drive operated at a plurality of rotating speeds has serious problems such as troubles in data recording and reproduction especially when one of the plurality of rotating speeds is much different another, e.g., twice or three times the other.
When data are written, the expanse of a magnetic field formed by the magnetic head can be made smaller, the smaller the gap between the recording medium and the magnetic head. The magnetized area of the recording medium can be thus kept small. That is, in order to increase the recording density of a magnetic disk drive, the gap between the recording medium and the magnetic head or the flying height of the magnetic head must be small. However, since a magnetic head is floated above a recording medium by an air pressure generated at an air bearing of a magnetic head slider, the flying height varies depending on the speed at which air flows into the gap between the magnetic head slider and the recording medium or the rotating speed of the recording medium. In general, the flying height of a magnetic head is greater at a higher rotating speed and smaller at a lower rotating speed. Therefore, when the air bearing is designed to prevent any contact in the flying at the lower rotating speed, problems can occur during recording or reproduction of data when the head flies at a great height at a high rotating speed because the gap between the recording medium and the magnetic head becomes to large. When elaboration is made to generate not only a positive pressure but also a negative pressure at the air bearing, the influence of the relative speed between the recording medium and the magnetic head on the flying height can be eliminated to some degree. However, when a difference between rotating speeds is as great as two or three-fold, there will be a speed difference of five-fold or more between the outer circumference of a disk rotating at the higher speed and the inner circumference of the same rotating at the lower speed. Resultant changes in the flying height can cause serious problems such as troubles in recording or reproduction of data.